sorting by strings in c++

There is a txt file that contains the following data:

• the name of a city
• a temperature in the city, measured in celsius

for example

London 3
Washington 11
Budapest 3
London 5
Budapest 30
Washington 11
London 15
Budapest 7

How can I calculate each city’s average temperature. Can I somehow sort it and create a structure by the city names, or what is the best solution for this issue?

>Solution :

std::map does everything for you – it aggregates by same key (city name) and also sorts all keys (city names), because std::map gurantees to keep all keys in sorted order.

In map I keep city name as key and pair of temperature sum and count as value. Finally average temperature can be computed as sum divided by count.

Try it online!

#include <fstream>
#include <map>
#include <iomanip>
#include <iostream>

int main() {
    {
        std::ofstream f("test.txt");
        f << std::string(
            "London 3 \n Washington 11 \n Budapest 3 \n London 5 \n"
            "Budapest 30 \n Washington 11 \n London 15 \n Budapest 7 \n");
    }
    {
        std::ifstream f("test.txt");
        std::map<std::string, std::pair<int, int>> temps;
        std::string name;
        int temp = 0;
        while (f >> name >> temp) {
            temps[name].first += temp;
            ++temps[name].second;
        }
        for (auto const & [name, avg]: temps)
            std::cout << name << ": " << std::fixed << std::setprecision(2)
                << 1.0 * avg.first / avg.second << std::endl;
    }
}

Output:

Budapest: 13.33
London: 7.67
Washington: 11.00